QL 
9.5,8 


UC-NRLF 


STUDIES  ON  THE  GERM  CELLS  OF  APHIDS 


BY  N.  M.  STEVENS 


WASHINGTON,  D.  C. 

Published  by  the  Carnegie  Institution  of  Washington 
May,  1906 


UNIVERSITY  FARM 


S8 


STUDIES  ON  THE  GERM  CELLS  OF  APHIDS 


BY  N.  M.  STEVENS 


WASHINGTON,  D.  C. 
Published  by  the  Carnegie  Institution  of  Washington 
May,  1906 


LIBRARY 
UNIVERSITY  OF  CALIFORNIA 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  51. 


FROM  THE  PRESS  OF 

THE  WILKENS-SHEIRY    PRINTING  CO. 

WASHINGTON,   D.  C. 


STUDIES  ON  THE  GERM  CELLS  OF  APHIDS. 


By  N.  M.  STEVKNS. 


INTRODUCTION. 

My  previous  report  on  the  germ  cells  of  aphids  was  based  on  a 
study  of  the  parthenogenetic  and  winter  eggs  of  Aphis  rosae,  the 
brown  rose  aphid,  and  of  the  spermatogenesis  of  Aphis  oenotherae, 
found  on  the  inflorescence  of  Oenothera  biennis. 

During  the  past  summer  and  autumn  material  from  more  than 
twenty  species  has  been  collected  and  examined.  In  some  cases  it 
has  been  possible  to  study  all  of  the  forms  of  one  species,  in  other 
cases  only  the  parthenogenetic  eggs,  only  the  winter  eggs,  or  only 
the  spermatogenesis. 

It  has  become  evident  in  the  course  of  the  work  that  little  depen- 
dence is  to  be  placed  on  the  present  classification  of  aphids,  and  that 
a  reclassification  based  on  the  cytology  of  the  germ  cells  may  be  nec- 
essary. For  example,  it  appears  that  at  least  three  distinct  species 
are  usually  included  under  the  name  Aphis  rosae.  No  attempt  will, 
therefore,  be  made  in  this  paper  to  give  the  L,atin  names  of  the  species 
studied,  but  they  will  be  designated  by  the  names  of  their  host  plants, 
and  identified  by  brief  description  where  it  seems  necessary. 

This  comparative  study  was  undertaken  in  the  hope  of  throwing 
more  light  on  the  question  of  sex  determination,  and  the  descriptive 
part  of  the  paper  will  be  followed  by  a  further  discussion  of  the  bear- 
ing of  the  facts  recorded  on  that  problem. 

METHODS. 

The  methods  used  were  for  the  most  part  the  same  as  in  the  pre- 
vious work — fixation  with  Gilson's  "acetic  alcohol  with  sublimate" 
(Lee,  sixth  edition,  p.  65)  and  staining  by  Heidenhain's  iron-hsema- 
toxylin  method.  Some  material  was  fixed  in  Flemming's  fluid, 
Hermann 'splatino  aceto-osmic,  and  Gilson's  mercuro-nitric,  but  such 
fixation  brought  out  no  new  points  in  structure,  and  was,  in  general, 
less  satisfactory  than  fixation  with  Gilson's  acetic  alcohol-sublimate 
formula.  The  latter  fluid  was  used  at  first  on  account  of  its  great 


4  STUDIES   ON   THE   GERM    CEIXS   OF   APHIDS. 

penetrating  power,  and  because  aphids  will  not  sink  in  any  water 
mixture;  but  by  opening  the  insects  with  needles  in  the  fluid  any 
method  of  fixation  may  be  used. 

Material  from  different  species,  though  fixed  in  the  same  way,  takes 
the  stain  so  differently  that  it  has  often  been  found  necessary  to 
restain  several  times  to  get  just  the  right  differentiation  for  counting 
and  drawing  the  chromosomes  accurately.  Schneider's  aceto-carmine 
proved  to  be  useful  in  determining  the  character  of  new  male  material. 

RESULTS  OF  INVESTIGATION. 
The  Rose  Aphids. 

On  the  roses  of  this  section,  both  out  of  doors  and  in  the  green- 
house, one  commonly  finds  a  brown  and  a  green  aphid  ;  and  on  the 
various  hardy  roses  there  appear  in  autumn  winged  mothers  of  another 
species  with  large  broods  of  white  or  yellowish  winter-egg  layers,  and 
a  little  later  the  winged  males  are  present.  The  host  from  which  this 
latter  species  migrates  to  the  rose  for  sexual  reproduction  has  not  yet 
been  discovered,  and  the  parthenogenetic  forms  have,  therefore,  not 
been  studied  ;  it  is,  however,  hoped  that  these  may  be  obtained  in  the 
spring  from  eggs  laid  on  isolated  rose  plants  in  the  greenhouse. 

In  my  last  year's  paper  (Stevens,  '05)  the  behavior  of  the  female 
germ  cells,  both  parthenogenetic  and  sexual,  of  the  brown  rose  aphid 
was  quite  fully  described  ('05,  pis.  i-ui,  figs.  1-27),  but  males  were  not 
found.  This  year  some  males  were  obtained,  but  they  were  very  few 
compared  with  the  number  of  sexual  females,  and  for  some  reason 
the  material  did  not  fix  so  well  as  that  from  most  of  the  other  species. 
A  few  good  drawings  of  the  first  spermatocyte  were  made  from  aceto- 
carmine  preparations  of  the  testes  taken  from  specimens  collected 
on  November  13  (pi.  I,  figs.  1—3).  The  metaphase  of  the  second 
spermatocyte,  from  a  section,  is  shown  in  figure  4.  Figure  5,  the 
metaphase  of  a  segmentation  spindle,  and  figure  6,  the  maturation 
mitosis  of  the  parthenogenetic  egg,  confirm  the  results  recorded  in  my 
earlier  paper,  showing  the  double  series  of  maternal  and  paternal 
chromosomes  in  the  parthenogenetic  generation.  No  favorable  mat- 
uration stages  of  the  winter  eggs  were  obtained.  The  scarcity  of 
males  was  accounted  for  by  the  discovery  of  male  and  female  embryos 
in  the  same  individual,  but  only  one  or  two  males  to  many  females. 
In  this  species  the  parthenogenetic  females  and  the  males  are  a  reddish 
brown,  while  the  sexual  females  are  red  and  the  eggs  when  first  laid 
are  green.  The  males  are  winged,  the  sexual  females  and  the  mothers 
of  the  sexual  generations  apterous. 


ROSE   APHIDS.  5 

The  green  rose  aphid  was  studied  last  year  only  in  the  partheno- 
genetic  form  and  the  winter  egg,  and  as  there  was  some  doubt  as  to 
the  number  of  chromosomes,  it  did  not  appear  in  the  published  results. 
As  in  the  case  of  the  brown  rose  aphid,  males  are  scarce,  but  some 
drawings  were  obtained  from  aceto-carmine  preparations  and  from  a 
few  good  sections.  The  number  of  chromosomes,  seven  in  the  sperm- 
atocyte  (figs.  7-9,  n,  13,  14),  shows  this  to  be  a  distinct  species,  as 
had  been  indicated  by  experiments  with  the  two  kinds  in  the  .green- 
house. All  of  the  forms  of  this  aphid  are  green,  and  the  freshly  laid 
eggs  are  colorless. 

Figures  7  to  9,  drawn  from  aceto-carmine  preparations  on  November 
14,  show  the  equatorial  plate  of  the  first  spermatocyte.  The  amount  of 
chromatin  may  not  vary  greatly  from  that  in  the  corresponding  cells 
of  the  brown  aphid,  but  it  is  differently  distributed.  Figure  10  is  a 
prophase  of  the  first  spermatocyte  mitosis  showing  the  side  to  side 
pairing  of  homologous  chromosomes.  Figure  n  is  a  metaphase  of 
the  first  spermatocyte  from  a  section,  and  figure  12  a  late  anaphase 
showing  the  lagging  pair  of  chromosomes — characteristic  of  the  first 
spermatocyte  spindle  in  all  of  the  species  studied.  Figures  13  and  14 
are  equatorial  plates  of  the  second  spermatocyte,  showing  the  same 
number  and  proportionate  size  of  chromosomes  as  in  the  first  sper- 
matocyte. In  figure  15  is  shown  the  double  series  of  14  chromosomes 
in  a  two-cell  stage  of  the  parthenogenetic  egg.  This  is  also  seen  in 
figures  i6a  and  i6£,  two  sections  of  the  germinal  vesicle  of  a  partheno- 
genetic egg  nearly  ready  for  maturation.  At  this  stage,  as  also  in  the 
prophase  of  segmentation  mitoses,  the  chromosomes  are  larger  and 
more  irregular  in  outline  than  in  the  equatorial  plate  of  the  spindle. 

The  third,  or  migratory  rose  aphid,  has  so  far  yielded  no  results 
except  from  the  winter  eggs,  which  have  been  laid  in  considerable 
numbers  on  isolated  rose  bushes  in  the  greenhouse.  Only  one  polar 
spindle  was  found  and  that  was  cut  diagonally.  The  nine  chromo- 
somes are  shown  in  figures  ija  and  17^;  these,  however,  together 
with  figures  18  and  19,  equatorial  plates  of  segmentation  spindles  of 
the  winter  egg,  show  the  number  and  size  relations  of  the  chromo- 
somes. The  two  broader  and  much  longer  chromosomes  are  without 
doubt  homologous  paternal  and  maternal  elements,  and  the  sixteen 
shorter  ones  can  easily  be  paired. 

Thus  it  appears  that  these  three  species  of  rose  aphids  are  as  dis- 
tinctly separated  by  differences  in  number,  form,  and  size  of  chromo- 
somes (figs.  5,  15,  1 8)  as  they  are  by  differences  in  external  character- 
istics of  form,  size,  and  color. 


6  STUDIES  ON  THE  GERM    CELLS  OF  APHIDS. 

The  Willow  Aphids. 

In  some  material  collected  by  Prof.  Morgan  from  willow  trees  at 
Saranac,  New  York,  early  in  September,  were  found  a  few  young  males 
and  mothers  of  the  sexual  generation.  Figures  2oa  and  2o£  are  two 
sections  of  a  first  spermatocyte  before  the  pairing  of  the  chromosomes, 
each  section  containing  one  complete  series  of  five.  Figure  21  is  a 
similar  stage  showing  the  two  largest  chromosomes  and  five  of  the 
smaller  ones.  Figure  22  shows  two  pairs  of  homologous  chromosomes 
from  a  slightly  later  stage  than  that  of  figures  20  and  21.  Figure  23 
is  the  metaphase  of  the  first  spermatocyte,  and  figure  24  shows  daugh- 
ter plates  of  the  anaphase.  In  figure  25  is  seen  a  pair  of  second  sper- 
matocytes  immediately  after  the  formation  of  the  nuclear  membrane. 
The  second  division  was  not  found,  but  a  drawing  of  the  young 
spermatid  is  given  in  figure  26.  The  number  of  chromosomes  here  is 
the  same  as  in  the  brown  rose  aphid,  but  the  proportionate  size  is 
somewhat  different.  There  is  more  difference  in  size  between  the 
largest  and  the  second  in  size,  and  the  two  smallest  are  proportion- 
ately larger  than  the  corresponding  ones  in  the  rose  aphid.  The 
mothers  of  the  sexual  generations  produce  offspring  of  both  sexes. 

Much  to  my  surprise,  material  collected  on  June  29  at  Harpswell, 
Maine,  contained  sexual  forms.  These  aphids  were  taken  from  the 
hairy,  reddish  young  shoots  of  one  of  the  dwarf  willows,  and  they 
could  not  have  been  many  generations  away  from  the  winter  egg,  for 
it  was  still  early  spring  on  the  coast  of  Maine.  There  were  a  few 
young  males  and  females,  and  the  apterous  parthenogenetic  mothers 
contained  embryos  in  the  following  combinations  : 

1.  All  parthenogetic. 

2.  All  sexual  female. 

.     3.  Parthenogenetic  and  sexual  female. 

4.  Parthenogenetic  male. 

5.  Parthenogenetic  male  and  female. 

Kyber  (1815)  mentions  finding  sexual  forms  on  the  willow  the  last 
of  June,  but  attributes  their  presence  to  the  change  in  sap  conditions 
of  the  host  at  the  close  of  the  growing  season,  while  in  this  case  they 
were  found  on  young  growing  shoots. 

The  polar  spindle  of  the  parthenogenetic  egg  in  metaphase  is  shown 
in  figure  27,  and  prophases  of  segmentation  spindles  in  figures  28  and 
29.  Six  chromosomes,  which  evidently  form  three  pairs,  are  present. 
Figure  30  shows  resting  cells  of  a  segmenting  parthenogenetic  egg. 
The  conspicuous  nucleolus  with  its  several  vacuoles  divides  into  two 
as  a  prophase  of  nuclear  and  cell  divisions.  Figures  31  and  32  are 
first  spermatocytes  in  metaphase  and  figures  33  and  34  in  anaphase; 


OENOTHERA  APHIDS.  7 

here  the  smaller  chromosome  appears  always  to  be  the  one  to  lag 
behind.  Figure  35  shows  the  partial  resting  stage  between  the  two 
spertnatocyte  mitoses.  No  second  spermatocytes  were  found  in  divi- 
sion. The  same  side  to  side  pairing  of  chromosomes  as  in  the  other 
species  was  observed. 

This  species  has  the  smallest  number  of  chromosomes  of  any  of 
those  studied,  and  it  apparently  has  the  smallest  amount  of  chroma- 
tin,  if  we  may  judge  by  comparison  of  the  metaphase  of  the  first 
spermatocyte  in  the  various  species. 

The  Oenothera  Aphids. 

As  the  spermatogenesis  of  Aphis  oenotherae  was  fully  described  last 
year  (Stevens  '05,  pi.  iv,  figs.  28-42),  only  two  new  figures  of  the  first 
and  second  spermatocyte  will  be  introduced  for  comparison  with 
otherspecieshavingthesamenumberofchromosom.es  (figs.  36  and 
37.)  The  size  relation  resembles  quite  closely  that  of  the  brown  rose 
aphid,  with  which  it  was  compared  in  my  former  paper,  but  the  largest 
chromosome  is  usually  more  elongated  and  the  difference  between  the 
two  largest  is  greater.  (Compare  figs.  3  and  36.)  In  external  appear- 
ance Aphis  oenotherae  is  very  different  from  Aphis  rosae.  The  females 
are  a  bright  red  and  the  males  and  their  apterous  mothers  green . 
This  aphid  was  not  found  on  the  young  plants  at  Bryn  Mawr  in 
spring,  or  on  the  Oenotheras  at  Harpswell,  Maine,  or  Woods  Hole, 
Massachusetts,  during  the  summer.  It  may  therefore  have  a  second 
host,*  from  which  it  migrates  to  Oenothera  biennis  in  the  late  summer 
or  autumn. 

Another  smaller,  green  aphid  was  found  in  abundance  on  the  foliage 
of  the  Oenotheras  at  Harpswell,  Woods  Hole,  and  Bryn  Mawr.  The 
parthenogenetic  form  of  this  aphid  is  light  green,  the  winter  egg-lay- 
ers darker  with  a  blackish  longitudinal  stripe  on  the  back,  and  the 
apterous  males  brownish  and  very  small.  This  was  the  only  species 
in  which  apterous  males  were  observed.  The  equatorial  plate  of  the 
maturation  spindles  of  the  parthenogenetic  egg,  with  eight  chromo- 
somes, is  shown  in  figure  38,  the  metaphase  of  the  first  spermatocyte 
in  figures  39  and  40,  the  anaphase  in  figures  41  and  42,  and  a  spermatid 
in  figure  43.  « 


*  Aphis  oenotherae  has  recently  hatched  in  considerable  numbers  on  rosettes  of 
Oenothera  biennis,  where  the  eggs  were  laid  in  the  greenhouse  in  October.  The  egg 
layers  were  red,  but  their  offspring  are  both  red  and  green,  and  individuals  of  either 
color,  isolated,  have  produced  offspring  of  the  same  color  in  the  second  and  third 
generations.  If  they  continue  to  breed  true  to  color  an  attempt  will  be  made  to  keep 
several  lines  of  each  color  isolated  until  the  sexual  generation  appears  in  the  autumn. 


8  STUDIES   ON   THE   GERM    CELLS   OF   APHIDS. 

The  Milkweed  Aphids. 

On  the  milkweeds  at  Woods  Hole,  only  one  kind  of  aphid  was 
found,  a  very  dark  colored  one — nearly  black — which  lived  in  herds 
attended  by  ants.  Sexual  forms  were  collected  on  September  27.  The 
metaphase  of  the  maturation  mitosis  of  the  parthenogenetic  egg  is 
shown  in  figure  44,  and  that  of  a  spindle  of  the  4-cell  stage  in  figure 
45.  The  prophase  of  a  spermatogonial  mitosis  appears  in  figure  46, 
exhibiting  the  same  double  series  of  chromosomes  as  in  the  parthen- 
ogenetic egg.  The  metaphase  of  the  first  spermatocyte  is  shown  in 
figure  47,  and  daughter  plates  of  the  anaphase  in  figure  48.  The 
lagging  chromosome  in  the  anaphase  (fig.  49)  is  here  the  smallest  of 
the  four.  Figures  50  and  51  show  different  views  of  the  anaphase  of 
the  second  spermatocyte.  The  number  of  chromosomes  is  the  same 
as  in  the  second  Oenothera  aphid,  but  the  difference  in  form  and  size 
is  evident.  (Compare  figs.  38  and  44,  39  and  47.) 

On  the  milkweeds  at  Bryn  Mawr  two  species  were  found  in  abun- 
dance, one  pale  (white  with  some  brown  spots),  and  the  other  a  bril- 
liant orange.  The  latter  remained  parthenogenetic  until  the  plants 
were  destroyed  by  frosts,  no  sexual  forms  appearing.  A  few  drawings 
were  made  from  the  parthenogenetic  egg.  Figures  52  and  53  are  the 
metaphase  of  the  maturation  spindle,  with  two  chromosomes  conspic- 
uously larger  than  the  other  six.  Figure  54  is  a  similar  stage,  but 
the  two  largest  chromosomes  are  evidently  united.  Two  equatorial 
plates  of  this  kind  were  found  in  embryos  of  one  mother,  while  the 
usual  number — eight — appeared  in  segmentation  stages  of  eggs  in  the 
same  embryos.  In  this  species  the  chromosomes  become  distinct  and 
take  stains  well  from  the  moment  when  the  growth  stage  of  the  oocyte 
begins.  A  young  oocyte,  just  leaving  the  ovary,  is  shown  in  figure 

55.  A  large  plasmosome  (p)  and  six  of  the  chromosomes  appear  in  this 
section.     A  prophase  of  mitosis  in  a  i6-cell  stage  is  shown  in  figure 

56,  and  a  section  of  a  4-cell  stage,  showing  the  polar  body  (pb]  and 
the  metaphase  of  three  spindles,  in  figure  57. 

In  the  spermatocytes  of  the  pale  milkweed  aphid  there  are  seven 
chromosomes,  'always  grouped  in  a  characteristic  way  (figs.  58  and 
59).  The  smallest  chromosome  is  closely  associated  with  the  next  in 
size  and  the  two  occupy  the  center  of  the  plate.  The  arrangement 
suggested  the  conditions  in  some  of  the  Hemiptera  heteroptera,  but 
careful  examination  of  the  anaphase  of  both  maturation  spindles 
showed  that  these  two  chromosomes  divide  like  the  others,  both,  how- 
ever, lagging  behind  the  others  (figs.  60  and  61).  Figure  62  is  a 


MILKWEED   AND    OAK   APHIDS.  9 

daughter  plate  of  the  first  spermatocyte.  Figures  63  and  64  are  slightly 
different  stages  in  the  pairing  of  the  chromosomes  as  a  prophase  of  the 
first  maturation  mitosis.  In  the  second  spermatocyte  the  two  smallest 
chromosomes  are  as  widely  separated  as  any  other  two  (fig.  65).  One 
individual  in  this  material  had  nine  instead  of  seven  chromosomes  in 
the  spermatocytes,  the  two  smallest  closely  associated  in  the  center 
of  the  plate  and  seven  in  the  outer  circle.  This  was  probably  a  stray 
specimen  of  another  species,  perhaps  closely  related,  for  here  the 
arrangement  of  the  chromosomes  seems  to  be  as  characteristic  of  the 
species  as  their  form  and  size.  One  day's  collection  of  this  material 
contained  several  individuals  which,  on  dissecting,  showed  both  em- 
bryos and  winter  eggs  from  the  same  ovaries,  thus  confirming  what  I 
found  once  among  my  rose  aphids  in  the  greenhouse;  but  the  fact 
that  I  collected  and  examined  a  great  deal  of  the  same  kind  of  mate- 
rial from  the  same  locality  on  the  following  days,  and  found  no  more 
cases  of  this  kind,  shows  it  to  be  of  rare  occurrence. 

In  the  black  and  the  orange  milkweed  aphids  we  have  two  conspic- 
uously different  aphids  with  chromosomes  of  the  same  number  and 
not  markedly  different  size  relations.  A  similar  looking  aphid  to  the 
black  milkweed  aphid  was  found  on  the  garden  nasturtium,  and  here 
the  number  and  size  of  the  chromosomes  is  near  enough  the  same  to 
make  it  probable  that  they  are  the  same  species.  Figures  66,  67,  and 
68  show  the  prophase  and  metaphase  of  the  first  spermatocyte  of  the 
nasturtium  aphid. 

The  Oak  Aphids. 

On  the  red  oak  were  found  some  peculiarly  marked  red  and  yellow 
aphids,  the  males  having  spotted  wings.  These  have  seven  chromo- 
somes in  the  spermatocytes  (fig.  69).  Figure  70  is  a  daughter  plate 
of  a  first  spermatocyte,  figure  71  an  equatorial  plate,  and  figure  72  a 
daughter  plate  of  a  second  spermatocyte.  One  specimen  in  this 
material  had  eight  chromosomes,  the  arrangement  being  similar  to 
that  in  the  pale  milkweed  aphids,  but  the  number  different  (figs.  73 
and  74). 

On  another  species  of  oak  was  found  a  white  aphid  with  seven  chro- 
mosomes, varying  considerably  in  size  from  those  of  the  red  oak 
aphid.  First  spermatocytes  are  shown  in  figures  75  to  78.  These  figures 
show  no  uniformity  of  grouping.  In  fact,  the  only  point  of  uniformity 
so  far  discovered  in  any  species  is  the  close  association  of  the  same 
two  chromosomes  in  the  first  spermatocytes  of  the  pale  milkweed 
aphids  and  the  one  stray  red  oak  aphid  (figs.  58  and  73). 


10  STUDIES   ON   THE  GERM    CELLS  OF  APHIDS. 

The  Goldenrod  Aphids. 

The  goldenrods,  besides  serving  as  hosts  for  several  beetles,  offer  to 
the  insect  collector  at  least  two  distinct  species  of  aphids.  On  the 
beach  goldenrod  are  large  numbers  of  dark  brown  aphids  with  green 
males.  The  sexual  forms  were  found  at  Woods  Hole  on  September 
27.  The  polar  body  (pb)  and  the  segmentation  nucleus  of  the  par- 
thenogenetic  egg  are  shown  in  figures  79  a  and  79  b,  the  metaphase 
of  the  first  spermatocyte  in  figures  80  and  81,  one  daughter  plate  of  a 
late  anaphase  in  figure  82,  the  metaphase  of  the  second  spermatocyte 
in  figure  83,  and  one  of  the  daughter  plates  of  an  anaphase  in  figure 
84.  In  this  species  four  of  the  chromosomes  are  of  nearly  the  same 
form  and  size,  the  other  two  being  very  much  smaller  and  one 
slightly  smaller  than  the  other.  This  was  the  first  aphid  in  which 
both  male  and  female  embryos  were  found  in  sections  of  the  same 
individual.  This  was  quite  unexpected,  as  in  the  forms  studied  last 
year,  particularly  the  Oenothera  aphid  and  the  migratory  rose  aphid, 
the  male  and  female  sexual  generations  seemed  to  be  entirely  separate, 
the  former  from  apterous,  the  latter  from  winged  mothers. 

The  tall  goldenrod,  Solidago  altissima,  is  the  host  of  quite  a  differ- 
ent aphid  with  the  same  number  of  chromosomes.  The  ordinary 
parthenogenetic  individuals  and  the  sexual  females  are  green,  the 
males  red,  and  the  mothers  of  the  sexual  generation  either  green  or 
red,  individuals  of  both  colors  giving  rise  to  both  male  and  female 
progeny.  Figures  85  and  86  are  prophases  of  the  first  spermatocyte 
mitosis,  figure  85  before  the  pairing  of  the  chromosome  and  figure  86 
during  that  process.  Figure  87  is  the  metaphase  and  figure  88  an 
early  anaphase  of  the  first  spermatocyte  from  material  fixed  with  Gil- 
son's  fluid,  figures  89  and  90  from  Flemming  material.  Figures  91 
and  92  are  the  metaphase  of  the  second  spermatocyte.  The  chromo- 
somes, though  the  same  in  number,  will  be  seen  to  differ  considerably 
from  those  of  the  brown  goldenrod  aphid  in  both  form  and  size. 

The  Clover,  Birch,  and  Beech  Aphids. 

On  the  paper  birch  were  found  the  sexual  forms  of  a  brown  aphid. 
Most  of  the  males  were  too  old  to  be  of  use  and  there  were  no  parthen- 
ogenetic forms  when  they  were  discovered  late  in  October.  Only 
one  drawing  of  the  first  spermatocyte,  showing  nine  chromosomes, 
was  made  from  an  aceto-carmine  preparation  (fig.  93).  Only  a  few 
young  males  of  the  clover  aphid  were  obtained.  The  testes  of  these 
showed  a  characteristic  metaphase  of  the  first  spermatocyte  contain- 
ing one  large  chromosome  surrounded  by  a  circle  of  seven  nearly 


STAR   CUCUMBER   AND    MAPLE   APHIDS.  II 

equal  in  size  (fig.  94).     Figures  95  and  96  are  prophases  of  the  first 
maturation  mitosis. 

The  first  and  second  spermatocytes  of  the  woolly  beech  aphid  have 
such  equatorial  plates  as  are  seen  in  figures  97,  98,  and  99.  The  ana- 
phase  (figs.  100  and  101)  is  rather  interesting,  as  here  the  lagging 
chromosome  is  evidently  the  largest. 

The  Star  Cucumber  Aphid. 

This  aphid  was  discovered  quite  late,  just  before  the  frost  killed  the- 
host  plant,  and  only  the  sexual  generation  with  the  mothers  of  the 
males  and  females  was  present.  The  males  were  winged  and  red, 
the  females  pale  green,  and  the  mothers  of  the  sexual  generation  were, 
some  red,  others  bright  green.  Here  again  the  same  mother  gives 
rise  to  both  sexes. 

Some  of  this  material  was  fixed  in  Hermann's  fluid,  and  figures 
102  to  105  were  taken  from  the  Hermann  material.  This  fixation  was 
no  better  for  the  mitotic  phases  than  that  obtained  with  the  Gilson 
formula,  but  the  nuclei  of  the  young  spermatocytes  showed  more  defi- 
nite structure  (figs.  102  and  103).  The  plasmosome  did  not  take  the 
haematoxylin  stain,  but  remained  a  yellowish  gray,  while  the  chro- 
matin  appeared  in  a  variously  tangled  and  wound  feathery  spireme. 
Figure  106  also  shows  the  metaphase  of  the  first  spermatocyte  and 
figure  107  an  early  anaphase.  Figure  104  is  a  prophase  showing  the 
paired  condition  of  one  of  the  longer  chromosomes,  and  figures  108 
and  109  are  pairs  of  daughter  plates  from  the  second  spermatocyte. 
In  this  aphid  we  have  a  marked  difference  in  the  size  relation  of  the 
chromosomes  from  that  in  the  other  species  that  have  the  same  num- 
ber. (Compare  figs.  3,  23,  36,  105  or  106). 

The  Maple  Aphid. 

This  is  a  pale  aphid — white  with  some  brown  markings.  Its  chief 
peculiarity  consists  in  the  large  number  of  chromosomes— sixteen  in 
the  spermatocytes.  As  shown  in  figure  no,  one  chromosome  is  al- 
ways much  larger  than  the  rest,  and  the  other  fifteen  vary  only  slight- 
ly in  size.  There  are  often  two  lagging  chromosomes  (fig.  in)  in  the 
anaphase  of  the  first  spermatocyte,  and  one  daughter  plate  is  usually 
concentrated  before  the  other,  giving  the  unsymmetrical  appearance 
of  figure  112.  In  figure  1 1 3 ,  a  prophase  of  the  first  spermatocyte  mi- 
tosis, paired  chromosomes  are  shown.  The  parthenogenetic  forms 
were  not  examined. 


12  STUDIES   ON   THE   GERM    CEUvS   OF   APHIDS. 

The  Pea  Aphid. 

The  pea  aphid,  or  pea  louse,  as  it  is  commonly  called,  shows  a 
very  neatly  paired  double  series  of  eight  chromosomes  in  the  parthen- 
ogenetic  generations.  Figure  114  is  from  a  maturation  spindle  and 
figures  115  and  116  from  a  32-cell  stage  of  the  parthenogenetic  egg. 
The  sexual  forms  were  not  available  for  study. 

The  Goumi  Aphid. 

On  the  shrub  known  as  the  goumi  is  found  an  interesting  aphid  of 
which  I  discovered  the  sexual  forms  too  late  to  get  anything  of  the 
spermatogenesis  or  to  find  the  parthenogenetic  forms.  The  males 
were  small  and  nearly  black;  the  females  of  three  distinct  colors — 
green,  red,  and  yellow — but  all  were  marked  alike  with  a  darker  stripe 
on  the  back.  When  brought  into  the  laboratory  they  laid  their 
orange-colored  eggs  in  abundance  on  a  white  cloth  tied  over  the  jar 
in  which  they  were  confined  to  prevent  their  traveling  all  over  the 
laboratory. 

Figures  117  and  118  show  polar  bodies  containing  five  chromo- 
somes, and  figure  119  the  male  and  female  pronuclei  in  contact,  the 
female  nucleus  being  the  larger  and  containing  two  plasmosomes  (p). 

Figures  120  a  and  120$  show  a  prophase  of  mitosis  from  an  8-cell 
stage.  Such  stages  are  of  course  not  so  good  for  comparison  of  the 
chromosomes  as  the  equatorial  plate,  but  the  two  series  of  homolo- 
gous chromosomes  are  fairly  well  shown,  considering  that  they  do 
not  all  lie  in  the  same  plane  or  even  exactly  parallel  with  the  surface 
of  the  section. 

In  many  cases  more  stages  might  have  been  figured,  but  it  did  not 
seem  advisable  to  multiply  figures  more  than  was  necessary  to  show 
the  characteristic  appearance  and  behavior  of  the  chromosomes  of 
each  species.  Several  drawings  of  the  same  stage  have  often  been 
given  to  show  the  variations  in  arrangement  of  the  chromosomes. 


SUMMARY   OK   RESULTS.  13 

SUMMARY  OF  RESULTS. 

1 .  Bach  species  of  aphids  is  characterized  not  only  by  a  specific 
number  of  chromosomes,  but  by  peculiarities  in  their  form  and  size, 
and  in  some  cases  by  a  definite  arrangement,  as  in  the  clover  aphid, 
the  pale  milkweed  aphids,  and  the  second  oak  aphid  described  (figs. 
58>  59 »  73 1  74>  94)-     Where  the  number  of  chromosomes  is  the  same 
in  two  species,  there  is  always  some  characteristic  difference  in  form 
or  size  corresponding  to  the  external  differences  in  the  species. 

2.  No  evidence  of  hybridism  has  been  observed  where  two  or  more 
species  occur  on  the  same  host. 

3.  There  is  no  evidence  of  any  reduction  in  the  number  of  chro- 
mosomes, or  of  more  than  one  maturation  mitosis  in  the  partheno- 
genetic  generations. 

4.  A  double  series  of  homologous  paternal  and  maternal  chromo- 
somes runs  through  the  parthenogenetic  generations,  and  the  homol- 
ogous chromosomes  are  paired  side  to  side  in  the  first  spermatocyte, 
and  presumably  in  the  oocyte,  of  the  sexual  generation  (figs.  10,  22, 
63,  64,  68,  86,  104,  113). 

5.  The  first  spermatocyte  mitosis  is  the  reducing  division,  sepa- 
rating homologous  chromosomes  paired  during  the  prophase. 

6.  There  are  no  heterochromosomes  of  any  kind,  but  the  sperma- 
tids  are  all  alike  so  far  as  number,  form,  and  size  of  chromosomes  is 
considered. 

7.  The  same  parthenogenetic  individual  may  produce  : 

(1)  All  parthenogenetic  embryos. 

(2)  Parthenogenetic  embryos  and  winter  eggs. 

(3)  All  sexual  female  embryos. 

(4)  All  male  embryos. 

(5)  Parthenogenetic  and  sexual  female  embryos. 

(6)  Parthenogenetic  and  male  embryos. 

(7)  Parthenogenetic,  male,  and  sexual  female  embryos. 

(8)  Male  and  sexual  female  embryos. 


14  STUDIES  ON  THE  GERM    CEU,S  OF  APHIDS. 

GENERAL  DISCUSSION. 
The  Relation  of  Species. 

If  we  may  suppose  that  all  the  species  of  aphids  having  the  same 
number  of  chromosomes  have  come  from  the  same  ancestors,  the 
mutations  may  be  indicated  by  such  differences  in  the  form  and  size 
relations  of  the  chromosomes  as  may  be  seen  by  comparing  the  first 
spermatocyte  of  the  brown  rose  aphid,  the  Saranac  willow  aphid,  the 
Oenothera  aphid,  No.  I,  and  the  star  cucumber  aphid,  with  five  chro- 
mosomes (figs.  3,  23,  36,  105);  or  that  of  the  black  milkweed  aphid, 
the  Oenothera  aphid  No.  II,  and  the  nasturtium  aphid  with  four  chro- 
mosomes (figs.  39,  47,  66);  or  again,  the  metaphase  of  the  maturation 
spindle  of  the  parthenogenetic  egg  of  the  orange  milkweed  aphid  and 
the  pea  aphid  with  eight  chromosomes  for  the  somatic  number  (figs. 
52,  114).  It  is  evident  that  mutations  might  as  easily  occur  by  a 
change  involving  the  number  of  chromosomes  as  their  form  and  size. 

The  three  species  having  the  two  smallest  chromosomes  closely 
associated  in  the  metaphase  of  the  first  spermatocyte  (figs.  58  and  73), 
but  differing  in  the  number  of  chromosomes — seven,  eight,  and  nine — 
were  similarly  marked  and  probably  closely  related. 

The  five  species  having  five  chromosomes  for  the  reduced  number 
were  obviously  different  in  form,  size, and  color,  while  of  those  having 
four  chromosomes  in  the  spermatocytes  only  the  black  milkweed 
aphid  and  the  nasturtium  aphid  were  nearly  enough  alike  to  suggest 
that  they  might  be  the  same  species,  though  one  was  found  on  milk- 
weed at  Woods  Hole,  Massachusetts,  and  the  other  on  garden  nastur- 
tiums at  Bryn  Mawr,  Pennsylvania. 

The  two  oak  aphids,  where  the  number  and  size  relations  of  the 
chromosomes  are  similar  (figs.  69  and  76),  were  very  unlike  in  appear- 
ance when  collected,  one  being  red  and  yellow,  the  other  nearly  white 
with  some  brown  dots  ;  but  the  alcoholic  specimens,  minus  their  col- 
oring, were  strikingly  alike. 

The  two  goldenrod  aphids,  with  the  same  number  of  chromosomes 
but  somewhat  different  form  and  size  relations  (figs.  80  and  87),  are 
extremely  unlike  in  appearance,  one  being  a  dark  reddish  brown  with 
green  males,  the  other  green  with  bright  red  males. 

This  examination  of  a  few  out  of  the  hundreds  of  species  of  aphids 
has  indicated  to  me  that  an  intelligent  classification  of  these  insects 
can  be  hoped  for  only  when  a  careful  study  of  the  external  characters 
and  habits  of  the  various  species  is  combined  with  an  equally  careful 
study  and  comparison  of  their  germ  cells.  My  work  can  be  regarded 
as  at  present  the  smallest  beginning  of  such  a  study,  and  is  merely 


GENERAL    DISCUSSION.  15 

suggestive  of  what  may  be  necessary  in  the  way  of  a  combination  of 
systematic  with  cytological  methods. 

Mendel's  Law  and  the  Individuality  of  the  Chromosomes. 

It  would  hardly  be  possible  to  find  better  material  than  the  germ 
cells  of  the  aphids  to  illustrate  the  individuality  of  the  chromosomes, 
and  the  probable  working  of  Mendel's  L,aw  of  Heredity. 

In  every  one  of  the  twenty-four  species  examined  some  or  all  of  the 
chromosomes  possess  characteristics  which  distinguish  them  from 
their  fellows,  and  these  peculiarities  persist  throughout  all  the  gen- 
erations. In  every  species  where  it  has  been  possible  to  study  and 
compare  the  germ  cells  of  the  parthenogenetic  and  sexual  generations, 
the  single  series  of  the  maturating  sexual  germ  cells  has  been  found  to 
be  exactly  duplicated  in  the  double  series  of  the  parthenogenetic  egg, 
the  segmenting  winter  egg,  and  the  spermatocytes  before  reduction; 
and  there  seems  to  be  no  room  for  doubt  that  homologous  maternal  and 
paternal  chromosomes  are  paired  and  then  separated  in  maturation. 

That  the  spermatids  must  be  "pure"  with  regard  to  the  paternal 
and  maternal  characters  represented  by  the  several  pairs  of  chromo- 
somes is  indicated  by  the  fact  that  in  the  spermatocyte  pairing  of  the 
chromosomes  does  not  occur  until  the  prophase  of  the  first  maturation 
mitosis,  and  even  then  the  homologous  chromosomes  are  merely 
paired  without  close  union.  There  is  therefore  little  opportunity  for 
mingling  of  the  chromatin  elements  of  any  two  paired  chromosomes. 

In  such  a  case  as  that  of  the  Harpswell  willow  aphid  with  only 
three  chromosomes  in  the  spermatocyte  there  must  be  extreme  corre- 
lation of  characters,  as  only  six  different  combinations  of  maternal 
and  paternal  chromosomes  would  be  possible  in  the  mature  eggs  and 
the  spermatozoa,  while  in  the  maple  aphid  with  sixteen  chromosomes, 
a  large  variety  of  different  combinations  of  maternal  and  paternal 
characters  is  possible.  Why  there  should  be  so  marked  a  difference 
in  number  of  chromosomes  and  in  amount  of  chromatin  in  the  several 
species  is  at  present  inexplicable.  I  thought  at  first  that  it  might 
be  possible  to  homologize  the  various  series,  and  see  where  a  single 
chromosome  in  one  species  might  be  equivalent  to  two  or  more  in 
another,  the  amount  of  chromatin  being  approximately  the  same  in 
all.  A  comparative  glance  at  the  plates  is  enough  to  show  that  such 
an  attempt  would  be  futile.  The  difference  in  amount  of  chromatin 
in  the  same  stage  of  maturation — for  example,  the  metaphase  of  the 
first  spermatocyte,  in  material  treated  in  exactly  the  same  way — 
though  not  so  great  as  the  difference  in  number,  is  still  very  evident. 


1 6  STUDIES   ON  THE  GERM    CELLS  OF  APHIDS. 

SEX   DETERMINATION. 

Though  this  study  was  undertaken  with  the  hope  that  it,  together 
with  experimental  work,  might  throw  considerable  light  on  the  much 
discussed  question  of  sex  determination,  it  has  yielded  more  abundant 
results  along  other  lines. 

The  discovery  of  embryos  and  winter  eggs  in  the  same  partheno- 
genetic  individual  has  been  confirmed,  but  shown  to  be  of  rare  occur- 
rence. However,  the  fact  that  this  can  occur  is  evidence  that  the 
parthenogenetic  and  winter  eggs  are  different  in  method  of  develop- 
ment, rather  than  in  their  original  constitution. 

Several  new  points  as  to  the  relation  of  the  sexual  to  the  partheno- 
genetic generations  have  been  gained  (Summary  of  Results,  No.  7). 
It  will  be  seen  at  once  that  these  relations  are  such  as  to  prove  con- 
clusively that  the  changes  in  sex  usually  attributed  to  changes  in  ex- 
ternal conditions  are  really  a  change  from  the  parthenogenetic  to  the 
sexual  mode  of  reproduction.  This  point  has  been  fully  discussed  by 
Castle  in  his  paper  on  the  Heredity  of  Sex,  page  190  ('03).  It  might 
still  be  maintained  that  different  external  conditions  had  caused  some 
eggs  to  develop  into  male  and  others  into  female  embryos  in  the  same 
individual,  had  not  embryos  been  repeatedly  found  of  different  sexes 
and  of  exactly  the  same  size,  and  presumably  of  too  near  the  same  age 
to  make  it  probable  that  the  developing  eggs  had  been  given  their  sex 
impulse  by  different  external  conditions.  Careful  examination  was 
made  to  see  whether,  in  cases  where  embryos  of  both  sexes  occurred, 
all  from  one  ovary  were  of  the  same  sex,  and  also  whether,  if  both 
sexes  come  from  the  same  ovary,  there  is  .any  definite  order  of  succes- 
sion in  the  oviducts.  These  are  not  easy  questions  to  answer  from 
sections  which  cut  the  embryos  in  various  planes,  but  it  was  ascer- 
tained that  both  sexes  occur  in  the  same  oviduct,  and  that  embryos  of 
different  sexes  may  alternate  or  two  or  more  successive  embryos  may 
be  of  the  same  sex. 

From  the  results  recently  obtained  in  the  study  of  the  germ  cells  of 
other  insects  [McClung  ('99-'o5),  Sutton  ('02,  '03),  Wilson  ('05) 
Stevens  ('05)  ],  it  now  seems  probable  that  the  sex  character  is  repre- 
sented in  the  chromatin  in  the  germ  cells  and  behaves  like  other  Men- 
delian  characters,  as  suggested  by  Castle  ('03).  Assuming  that  this 
is  true,  the  spermatozoa  must  be  dimorphic  with  respect  to  the  sex 
character — one  half  of  them  containing  the  male  character,  the  other 
half  the  female  character — since  the  mothers  of  the  males  may  pro- 
duce both  male  and  female  offspring,  and  there  is  no  reduction  of 
chromosomes  in  the  eggs  which  develop  into  male  embryos. 


GENERAL    DISCUSSION.  17 

It  is  evident  that  every  fertilized  aphid  egg  must  contain  the  female 
sex  character,  since  it  invariably  produces  a  parthenogenetic  female. 
The  possible  combinations  of  parthenogenetic,  sexual  female,  and  male 
embryos  cited  in  No.  7  of  the  Summary  of  Results,  page  13,  show  that 
in  all  probability  every  parthenogenetic  female  is  capable  of  pro- 
ducing sexual  female  progeny;  in  other  words,  the  female  character  is 
dimorphic,  if  one  may  so  describe  it.  It  gives  rise  under  some  condi- 
tions to  the  development  of  a  parthenogenetic  female,  under  other 
conditions  to  a  sexual  female.  Now  it  remains  to  be  determined 
whether  (i)  each  fertilized  egg  also  contains  the  male  character  re- 
cessive (Castle,  '03) ,  or  (2)  whether  only  a  part  of  the  eggs  contain  the 
male  and  female  characters  as  alternates,  while  the  remainder  have 
the  female  character  in  both  the  maternal  and  paternal  series  of  chro- 
mosomes. The  former  condition — male  and  female  characters  in  all 
eggs— rwould  imply  maturation  of  the  winter  eggs  according  to  the 
law  of  chances  and  meeting  in  fertilization  of  only  eggs  and  sperma- 
tozoa containing  different  sex  characters  (Castle,  '03).  The  latter 
condition  might  be  the  result  of  such  a  differential  maturation  of  the 
eggs  that  all  should  contain  only  the  female  sex  character  before  fer- 
tilization. The  sex  possibilities  of  the  fertilized  egg  would  then  be 
determined  by  the  entrance  either  of  a  spermatozoon  containing  the 
male  character  or  of  one  containing  the  female  character.  An  egg 
fertilized  by  a  spermatozoon  containing  the  female  character  would 
then  produce  a  line  of  aphids  consisting  only  of  parthenogenetic 
females  and  sexual  females.  An  egg  fertilized  by  a  spermatozoon  con- 
taining the  male  element  would  be  capable  of  producing  a  succession 
of  generations  consisting  of  parthenogenetic  females,  sexual  females 
and  males.  This  is  just  what  we  find  (Summary,  No.  7),  but  the  same 
conditions  could  occur  if  all  the  fertilized  eggs  contain  both  male  and 
female  characters. 

The  fact  that  in  some  species  (Oenothera  aphid  No.  I  and  the  white 
rose  aphid)  the  two  sexes  are  entirely  separate  in  the  sexual  genera- 
tion, and  that  in  the  six  species  in  which  embryos  of  both  sexes  are 
found  together  there  are  always  many  individuals  that  contain  only 
female  embryos,  suggested  the  possibility  that  there  might  be  in  each 
species  entirely  separate  lines  from  the  fertilized  egg  through  the  par- 
thenogenetic generations  to  the  sexual  generation,  some  lines  being 
capable  of  producing  males,  others  not.  The  only  way  to  test  this 
point  would  be  to  isolate  parthenogenetic  individuals  of  a  species, 
either  directly  from  the  fertilized  egg  or  from  somewhat  widely  sepa- 
rated hosts,  and  examine  the  sexual  generations  from  each.  If  many 
individuals  from  some  lines  of  descent  should  give  only  sexual 


1 8  STUDIES  ON  THE  GERM   CELLS  OF  APHIDS. 

females,  while  those  from  other  lines  gave  either  only  males  or  both 
males  and  females,  the  natural  conclusion  would  be  that  some  fertil- 
ized eggs  contain  the  male  character,  while  others  do  not.  If,  on  the 
other  hand,  both  males  and  females  should  occur  in  all  the  lines  of 
descent,  we  should  feel  reasonably  sure  that  all  the  fertilized  eggs 
contain  both  the  male  and  female  character. 

Unfortunately  there  is  no  such  visible  dimorphism  in  the  sperma- 
tids  of  the  aphid  as  in  some  of  the  Hemiptera,  Orthoptera,  and  Coleop- 
tera  to  serve  as  direct  evidence  of  sex -determining  power,  but  if  the 
sex  characters  are  represented  in  the  chromatin  as  Mendelian  alter- 
nates there  is  abundant  evidence  that  the  spermatogonia  must  con- 
tain both  characters  and  that  the  spermatozoa  must  be  pure  as  to  the 
two  alternating  characters. 

As  to  which  seems  the  more  likely  supposition  (i)  that  only  germ 
cells  with  opposite  sex  characters  can  unite  to  give  a  fertile  egg,  or 

(2)  that  all  the  winter  eggs  can  be  so  maturated  that  only  the  female 
character  remains,  one  proposition  seems  about  as  likely  to  be  true  as 
the  other  ;  and  at  present  I  see  no  third  possibility,  since  there  is  no 
evidence  that  the  female  element  is  removed  from  the  parthenogenetic 
eggs  that  produce  males  (Castle, '03)  and  the  spermatozoa  must  there- 
fore be  of  two  kinds  respecting  the  sex  characters,  just  as  in  the  case 
of  any  other  two  homologous  maternal  and  paternal  characters.* 

Whatever  evidence  isolation  experiments  may  give  on  this  point  ot 
the  distribution  of  the  male  and  female  characters  in  the  fertilized  egg, 
we  know  that  from  one  parthenogenetic  egg  can  come  an  individual 
whose  germ  cells  can  produce  (i)  parthenogenetic,  (2)  sexual  female,  or 

(3)  male  offspring,  and  so  far  as  we  can  see  the  indications  are  that 
the  same  influences  that  cause  a  change  in  the  mode  of  reproduction 
from  parthenogenetic  to  sexual  produce  in  some  species  or  in  some 
individuals  of  a  species  a  definite  change  in  dominance  of  the  sex 
characters  (Oenothera   aphid  No.  I   and  white  rose  aphid),  while  in 
other  species  (goldenrod  and  willow  aphids)  a  more  or  less  balanced 
state  of  dominance  must  be  associated  with  the  initiation  of  sexual 


*  In  the  case  of  Aphis  oenotherae  it  is  possible  that  the  germ  cells  of  the  red 
parthenogenetic  individuals  contain  the  female  sex  character  in  both  the  paternal 
and  the  maternal  series  of  chromosomes,  while  the  green  individuals  have  the  female 
sex  character  in  the  maternal  series  and  the  male  in  the  paternal  series,  the  green 
color  being  correlated  with  the  male  sex  character  and  dominating  over  the  maternal 
red.  The  former  line  would  then  give  rise  to  sexual  females  and  winter  eggs  con- 
taining only  the  female  character;  the  latter,  by  a  change  in  dominance  of  the  sex 
character,  would  produce  males  containing  both  characters.  Fertilization  by  the 
resulting  two  classes  of  spermatozoa  would  then  perpetuate  the  two  sexual  lines. 


GENERAL    DISCUSSION.  19 

reproduction  and  some  unknown  internal  factor  determine  which  sex 
character  shall  control  the  development  of  each  egg.  The  whole 
question  of  sex  determination  will  be  seen  to  be  far  more  complex  in 
the  case  of  the  aphids  than  in  insects  where  no  parthenogenetic  gener- 
ations intervene  between  the  fertilized  egg  and  the  production  of 
males. 

In  collecting  aphids  this  year  it  was  found  that  the  sexual  genera- 
tion appeared  in  many  species  from  two  to  three  weeks  earlier  than  in 
1903  and  1904,  though  the  weather  continued  warm  much  later  than 
during  the  two  preceding  autumns.  This  seemed  to  be  related  to  the 
earlier  ripening  up  of  the  various  host  plants.  The  earlier  fruiting 
and  drying  up  was  especially  conspicuous  in  Oenothera  biennis  and 
some  of  the  rose  bushes.  Sexual  forms  of  both  the  brown  and  green 
rose  aphid  were  found  on  young  growing  tips  of  these  same  bushes, 
but  they  may  have  been  affected  by  the  general  sap  conditions  of  the 
plant.  Finding  sexual  forms  among  the  willow  aphids  collected  from 
young  shoots  at  Harpswell,  Maine,  on  June  29,  suggested  that  in 
some  species  there  may  be  definite  cycles  of  generations  not  directly 
subject  to  external  conditions,  as  Weismann  maintains  for  the  Daph- 
nids  ('76,  '79).  This  species  of  aphids  deserves  further  investigation. 

One  is  also  somewhat  mystified  by  the  fact  that  in  some  species 
there  is  a  sudden  and  complete  change  from  the  parthenogenetic  to 
the  sexual  mode  of  reproduction,  while  in  others,  as  for  example  the 
brown  and  the  green  rose  aphids,  parthenogenetic  generations  continue 
to  multiply  until  destroyed  by  the  cold  of  winter.  The  complete 
change  seems  to  be  the  rule  in  the  majority  of  the  species  in  this  cli- 
mate, and  is  much  more  easily  explained  than  the  mixed  condition. 

So  far  it  has  not  been  possible  to  produce  the  sexual  generations 
by  subjecting  the  aphids  to  artificial  changes  of  conditions.  This 
may  be  due  to  the  fact  that  the  brown  and  green  rose  aphids,  which 
have  been  used  for  most  of  the  experiments,  may  be  the  most  unfavor- 
able for  such  work,  as  they  produce  scattered  sexual  forms  while  con- 
tinuing the  parthenogenetic  generations,  indicating  either  that  they 
are  less  sensitive  to  changes  in  environment  or  that  they  are  less  well 
adapted  to  this  climate. 

This  is  evidently  a  case  where  experimental  work  must  be  based  on 
rather  extensive  observation  and  cytological  research,  and  the  results 
recorded  in  this  paper  give  a  far  better  basis  for  experiment  than  we 
had  a  year  ago. 

BRYN  MAWR  COLLEGE,  January  75,  1906. 


BIBLIOGRAPHY.  21 


BIBLIOGRAPHY. 


CASTLE,  W.  E. 

"03.  The  heredity  of  sex.  Bull.  Mus.  Comp.  Zool.  Harvard  College,  vol.  40,  00.4. 
KYBER,  J.  F. 

'15.  Einige  Erfahrungen  und  Bemerkungen  iiber  Blattlause.  Germar's  Magazin 
der  Entomologie,  t.  i-ac. 

McCLUNG,   C.   E. 

'99  A  peculiar  nuclear  element  in  the  male  reproductive   cells  of   insects.    Zool. 

Bull.,  vol.  2. 
'oo.  The  spermatocyte  divisions  of  the  Acridiidae.  Kans.    Univ.    Quart.,    vol.    9, 

no.  i. 

'01.  Notes  on  the  accessory  chromosomes.     Anat.  Anz.,  bd.  20,  nos.  8  and  9. 
"02.  The  accessory   chromosome — Sex-determinant?    Biol.    Bull.,   vol.    3,    nos. 

i  and  2. 
'oza.  The  spermatocyte  divisions  of  the  Locustidas.  Kans.  Univ.  Quart.,  vol.  i, 

no.  8. 
'05.  The  chromosome  complex  of  orthopteran  spermatocytes.  Biol.  Bull.,  vol.  9, 

no.  5. 
STEVENS,  N.  M. 

'05.  A  study  of  the  germ  cells  of  Aphis  rosae  and  Aphis   oenotherae.   Journ.    of 

Exp.  Zool.,  vol.  2,  no.  3. 
'osa.  Studies  in   spermatogenesis,    with   especial    reference   to   the    "accessory 

chromosome".    Carnegie  Inst.  of  Wash.,  pub.  no.  36. 
SUTTON,  W.  S. 

'02.  On  the  morphology  of  the  chromosome  group  in  Brachystola  magna.   Biol. 

Bull.,  vol.  4,  no.  i. 

'03.  The  chromosomes  in  heredity.   Biol.  Bull.,  vol.  4,  no.  5. 
WEISMANN,  A. 

'76-79.  Beitrage  zur  Naturgeschichte  der  Daphnoiden.    Leipzig. 
WILSON,  E.  B. 

'05.  The  behavior  of  the  idiochromosomes  in  Hemiptera.    Journ.  of  Exp.  Zool., 

vol.  2,  no.  3. 

'o5«.  Studies  on  chromosomes.  II.  The  paired  microchromosomes,  idiochromo- 
somes, and  heterotropic  chromosomes  in  Hemiptera.  Journ.  of  Exp.  Zool., 

vol.  2,  no.  4. 


22  DESCRIPTION   OF  PLATES. 


DESCRIPTION  OF  PLATES. 


[The  figures  were  all  drawn  with  camera  lucida,  Zeiss  oil-immersion  2  mm.,  oc.  12 
and  have  been  reduced  one-third,  giving  a  magnification  of  1000  diameters.] 

PLATE  I. 
Brozvn  rose  aphid. 

FIGS.  1-3.  First  spermatocyte,  metaphase,  aceto-carmine  preparation  of  November  13. 

4.  Second  spermatocyte,  metaphase. 

5.  Parthenogenetic  egg,  metaphase  of  segmentation  spindle. 

6.  Parthenogenetic  egg,  metaphase  of  maturation  spindle.     The  double  series 

of  chromosomes  is  numbered,  here  and  in  other  figures,  in  order  of  size. 

Green  rose  aphid. 

7-9.  First  spermatocyte,  metaphase,  aceto-carmine  preparation  of  November  14. 

10.  First  spermatocyte,  prophase,  showing  paired  chromosomes. 

11.  First  spermatocyte,  metaphase,  from  section. 

12.  First  spermatocyte,  late  anaphase. 
13-14.  Second  spermatocyte,  metaphase. 

15.  Two-cell   stage   of  parthenogenetic    egg,  metaphase,    showing   the   double 
series  of  14  somatic  chromosomes. 

16  a  and  b.  Parthenogenetic  egg,  prophase  of  maturation  mitosis  showing   14 

chromosomes. 

Migratory  rose  aphid. 

17  a  and  b.  Winter  egg,  first  maturation  mitosis,  metaphase,  9  chromosomes. 
18-19.  Winter  egg,  metaphase  of  segmentation  spindles,  18  chromosomes. 

Saranac  zvillozv  aphid. 

20  a  and  b.  First  spermatocyte,  prophase,  showing  double  series  of  10  chro- 
mosomes. 

21.  First  spermatocyte,  prophase. 

22.  Paired  chromosomes  from  prophase  of  first  spermatocyte. 

23.  First  spermatocyte,  metaphase. 

24.  First  spermatocyte,  anaphase. 

25.  A  pair  of  second  spermatocytes. 

26.  Young  spermatid. 

Harpszuell  zvilloiv  aphid. 

27.  Parthenogenetic  egg,  metaphase  of  maturation  spindle. 
28-29.  Parthenogenetic  egg,  prophases  of  segmentation  mitosis. 


STEVENS 


PLATE     I. 


S?          * 

•.  i    *^ 


*d,.   ... 

f-.J 


•      vs. 

1  *  * 


* 


...• 


i6.b 


'*'f 

'  * 


20  b 


25- 


26. 


27. 


23. 


28. 


N.  M.  8.  del. 


24  DESCRIPTION   OF   PLATES. 


PLATE  II. 
Harp  swell  ivillozu  aphid. 

FIG.     30.  Parthenogenetic  egg,  resting  nuclei,  showing  nucleolus  divided  before 

mitosis. 

31-32*  First  spermatocyte,  metaphase. 
33-34.  First  spermatocyte,  anaphase. 

35.  Second  spermatocyte,  prophase. 

Aphis  oenotherae. 

36.  First  spermatocyte,  metaphase. 

37.  Second  spermatocyte,  metaphase. 

Oenothera  aphid,  No.  II. 

38.  Parthenogenetic  egg,  metaphase  of  maturation  spindle. 
39-40.  First  spermatocyte,  metaphase. 

41.  First  spermatocyte,  early  anaphase. 

42.  First  spermatocyte,  late  anaphase. 

43.  Young  spermatid. 

Black  milkweed  aphid. 

44.  Parthenogenetic  egg,  metaphase  of  maturation  spindle. 

45.  Parthenogenetic  egg,  4-cell  stage,  metaphase  of  segmentation  spindle. 

46.  Spermatogonium,  prophase  of  mitosis. 

47.  First  spermatocyte,  metaphase. 
48-49.  First  spermatocyte,  anaphase, 
50-51.  Second  spermatocyte,  anaphase. 

Orange  milkzveed  aphid. 

52-53.  Parthenogenetic  egg,  metaphase  of  maturation  spindle. 

54.  Parthenogenetic  egg,   metaphase  of  maturation  spindle,  showing  the 

two  largest  chromosomes  united  (i-j-i). 

55.  Young  oocyte,  showing  plasmosome  (p)  and  six  of  the  chromosomes. 

56.  Parthenogenetic  egg,  i6-cell  stage,  prophase  of  mitosis. 

57.  Parthenogenetic  egg,  polar  body  (p.  b.}  and  three  segmentation  spindles. 


STEVENS 


PLATE     II. 


«o        0 


* 


/ 


• 


<  '5S*3     *-tft-t 

1    ;  ''  Njp         ^  ~ 

38.  39.  ~  &: 


^    .X 
44- 


> — - 

49. 


50. 


* 


5'- 


5.?. 


55- 


56. 


57. 


N.  M.  8.  del. 


26  DESCRIPTION   OF   PLATES. 


PLATE  III. 
Pale  milk-weed  aphid, 

FIGS.  58-59.  First  spermatocyte,  metaphase. 
60-61.  First  spermatocyte,  anaphase. 

62.  First  spermatocyte,  daughter  plate. 
63-64.  First  spermatocyte,  prophase.  showing  paired  chromosomes. 

65.  Se"ccnd  spermatocyte,  metaphase. 

Nasturtium  aphid. 

66.  First  spermatocyte,  metaphase. 
67-68.  First  spermatocyte,  prophase. 

Oak  aphids. 

69.  No.  I,  first  spermatocyte,  metaphase. 

70.  No.  I,  first  spermatocyte,  anaphase. 

71.  No.  I,  second  spermatocyte,  metaphase. 

72.  No.  I,  second  spermatocyte,  anaphase. 
73-74.  No.  II,  first  spermatocyte,  metaphase. 
75-77.  No.  Ill,  first  spermatocyte,  metaphase. 

78.   No.  Ill,  first  spermatocyte,  anaphase. 

Beach  goldenrod  aphid. 

79  a  and  b.  Parthenogenetic  egg,  polar  body  and  prophase  of  first  segmen- 
tation mitosis. 
80-8 1.  First  spermatocyte,  metaphase. 

82.  First  spermatocyte,  anaphase, 

83.  Second  spermatocyte,  metaphase. 

84.  Second  spermatocyte,  anaphase. 

Tall  goldenrod  aphid. 

85.  First  spermatocyte,  before  pairing  of  chromosomes. 

86.  First  spermatocyte,  prophase,  showing  paired  chromosomes. 

87.  First  spermatocyte,  metaphase,  Gilson. 

88.  First  spermatocyte,  early  anaphase,  Gilson. 

89.  First  spermatocyte,  metaphase,  Flemming. 

90.  First  spermatocyte,  early  anaphase,  Flemming. 
91-92.  Second  spermatocyte,  metaphase. 


STEVENS 


PLATE     III. 


64. 


66. 


67. 


68. 


70. 


'» 


%\ 


73' 


«• 

V 

•"'  75. 


7*. 


78. 


79-rt 


79-* 


to. 


s.      /,-3" 

/»2«wJ      f  && ' 


N.  M.  S.  del. 


28  DESCRIPTION   OF   PLATES. 


PLATE  IV. 

Paper  birch  aphid. 
FIG.       93.  First  spermatocyte,  metaphase. 

Clover  aphid. 

94.   First  spermatocyte,  metaphase. 
95-96.  First  spermatocyte,  prophase. 

Woolly  beech  aphid. 

97-98.  First  spermatocyte,  metaphase. 

99.  Second  spermatocyte,  metaphase. 
TOO-IOI.  First  spermatocyte,  anaphase. 

Star  cucumber  aphid. 

102-103.   First  spermatocyte,  growth  stage. 

104.  First  spermatocyte,  prophase. 

105.  First  spermatocyte,  metaphase,  Hermann  preparation. 

106.  First  spermatocyte,  metaphase,  Gilson. 

107.  First  spermatocyte,  early  anaphase. 

108   a  and  b.   Second  spermatocyte,  anaphase. 
109.   Second  spermatocyte,  anaphase. 

Maple  aphid. 

no.  First  spermatocyte,  metaphase. 
1 1 1- 1 12.  First  spermatocyte,  anaphase. 

113.  First  spermatocyte,  prophase. 

Pea  aphid. 

114.  Parthenogenetic  egg,  metaphase  of  maturation  spindle. 

115-116.  Parthenogenetic  egg,  32-cell  stage,  metaphase  of  segmentation  spindle. 

Goumi  aphid. 

117-118.  Winter  egg,  polar  bodies. 

119.   Winter  egg,  male  and  female  pronuclei. 

1 20   a  and  b.  Winter  egg,  prophase  of  segmentation  mitosis. 


STEVENS 


PLATE     IV. 


93- 


* 

95- 


* 


/-""-x 

I      I 


70.?. 


io8.a 


104. 


105. 


io8b 


106. 


•••  m 


114. 


116. 


:'  ' 
I 


N.  M.  S.  del. 


MHo«nJ.CnB«lti.iiore 


UNIVERSITY    OF    CALIFORNIA 
BRANCH    OF    THE    COLLEGE    OF    AGRICULTURE 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


^621-58 
JAN  2  3  1961 
DEC  1  6  1966 


5m-8,'26 


"23 

SL95S 

O  D 

Stevens 

38 

Studies  o 

n  the  germ  ce 

Us 

of  aphids    • 

JAN  2  3  19 

51 

S'8- 


LIBRARY,  BRANCH  OF  THE  COLLEGE  OF  AGRICULTURE 


